1. Field of the Invention
The present invention relates to a differential amplifier, and more particularly, to a technique for reducing an input capacitance of a differential amplifier.
2. Descriptions of the Background Art
A. Background Art
FIG. 6 is a circuitry diagram showing a structure of a conventional differential amplifier. The differential amplifier 200 comprises input terminals 2a and 2b, output terminals 2c and 2d, an input circuit 20 and an output circuit 50. The input circuit 20 includes NPN transistors Q1 and Q2 and a constant current source 1a. The output circuit 50 includes resistors 11 and 12.
The transistors Q1 and Q2 disposed in the input circuit 20 are designed identical to each other. Emitters of the transistors Q1 and Q2 are commonly connected to the constant current source 1a and grounded through the constant current source 1a. Bases of the transistors Q1 and Q2 are commonly connected to the input terminals 2a and 2b, respectively. The constant current source 1a carries a constant current I.sub.a.
The resistors 11 and 12 disposed in the output circuit 50 equally have a resistance R, and one ends of the resistors 11 and 12 are commonly connected to a power source terminal 3. The power source terminal 3 receives a potential V.sub.C from a power source which is disposed external to the differential amplifier 200. The other end of the resistor 11 is connected to a collector of the transistor Q1 and the output terminal 2c, while the other end of the resistor 12 is connected to a collector of the transistor Q2 and the output terminal 2d.
A potential is supplied to the input terminals 2a and 2b from outside of the differential amplifier 200. For example, as shown in FIG. 6, a coil 10 is connected between the input terminals 2a and 2b. A Voltage is induced across the coil 10, and as a result, potentials V.sub.2a and V.sub.2b are supplied to the input terminals 2a and 2b, respectively.
Assuming here that the coil 10 is such a coil which is used as a head of a hard disk drive, for instance, as a magnetic member 90, i.e., the hard disk moves near the coil 10 as indicated by an arrow, a current flows through the coil 10. This current flowing in the coil 10 in turn develops a very small voltage as low as about 1m V.sub.pp across the coil 10.
The direction of the current which flows in the coil 10 changes depending on the direction of the movement and the polarity of the magnetic member 90. In accordance with the current value of this current, a corresponding amount of voltage is developed across the coil 10. The polarity of the voltage induced across the coil 10 changes depending on the direction of the current. In hereinafter describing an operation of the differential amplifier 200, the voltage thus induced is conceived to have a positive polarity when a potential at the input terminal 2a is higher than that at the input terminal 2b and to have a negative polarity when a potential at the input terminal 2a is lower than that at the input terminal 2b.
(A-1) Case Where There is No Potential Difference Between the Input Terminals 2a and 2b
In this case, the input terminals 2a and 2b are at the same potential, and therefore, a base-emitter voltage V.sub.BE1 of the transistor Q1 is equal to a base-emitter voltage V.sub.BE2 of the transistor Q2. Since the emitters of the transistors Q1 and Q2 have the same configuration and the same surface area, a current passed by the emitter of the transistor Q1 is equal to a current passed by the emitter of the transistor Q2. In each emitter, the value of the current flowing therein is I.sub.a /2, i.e., a half of the current value I.sub.a of the current which is supplied by the constant current source 1a.
Here, if the potential V.sub.C is set such that the collectors of the transistors Q1 and Q2 are at higher potentials than the bases of the transistors Q1 and Q2, in other words, such that the transistors Q1 and Q2 both operate in a non-saturation region, base currents in the transistors Q1 and Q2 can be ignored with respect to the emitter currents since the base currents are smaller than the emitter currents in the transistors Q1 and Q2. It then follows that each emitter current can be regarded as the current which flows from the associated collector.
Hence, a potential V.sub.2c available at the output terminal 2c is smaller than the potential V.sub.C which is provided to the power source terminal 3 by a multiplication product of the emitter current of the transistor Q1 and the resistance R of the resistor 11. Similarly, a potential V.sub.2d supplied to the output terminal 2d is smaller than the potential V.sub.C by a multiplication product of the emitter current of the transistor Q2 and the resistance R of the resistor 12. Since the currents flowing through the transistors Q1 and Q2 are both equally I.sub.a /2, the potentials V.sub.2c and V.sub.2d are both equally V.sub.C -(I.sub.1 /2).multidot.R.
FIG. 7 is a waveform diagram showing potentials at the respective parts of the differential amplifier 200. The exemplary situation described under (A-1) occurs at time t.sub.0 in FIG. 7.
(A-2) Case Where There is a Positive Potential Difference Between the Input Terminals 2a and 2b
As the situation described under (A-1) changes and a positive potential difference is created between the input terminals 2a and 2b, that is, when the potential V.sub.2a is raised above the potential V.sub.2b, the base-emitter voltage V.sub.BE1 of the transistor Q1 becomes greater than the base-emitter voltage V.sub.BE2 of the transistor Q2. As this occurs, the current flowing into the emitter of the transistor Q1 grows and the current flowing into the emitter of the transistor Q2 reduces.
This is because the emitters of the transistors Q1 and Q2 are commonly connected to the constant current source 1a and the total current value of the emitter currents of the transistors Q1 and Q2 is fixed at the constant current value I.sub.a.
The potential V.sub.2c drops from the value V.sub.C -(I.sub.a /2).multidot.R by a multiplication product of the increase in the emitter current of the transistor Q1 and the resistance R, whereas the potential V.sub.2d increases from the value V.sub.C -(I.sub.a /2).multidot.R by a multiplication product of the drop in the emitter current of the transistor Q2 and the resistance R. For instance, where the potentials V.sub.2a and V.sub.2b each change by 0.5 mV and the gain of the differential amplifier 200 is about 30, changes in the potentials V.sub.2c and V.sub.2d are about 15 mV.
Although changing by the same amount, the potentials V.sub.2c and V.sub.2d change in different directions. If the potential difference between the potentials V.sub.2c and V.sub.2d is amplified in the manner described above, when the potentials V.sub.2a and V.sub.2b are about 1 mV apart from each other, a potential difference between the potentials V.sub.2c and V.sub.2d will be amplified to about 30 mV. This corresponds to time t.sub.1 in FIG. 7.
(A-3) Case Where There is a Negative Potential Difference Between the Input Terminals 2a and 2b
This situation occurs when the transistors Q1 and Q2 operate in an opposite manner in the case (A-2). That is, the potential V.sub.2c is greater than the potential V.sub.2d by about 30 mV. In this situation, the potentials V.sub.2a and V.sub.2b have opposite polarities as shown in FIG. 7. If the potentials V.sub.2a and V.sub.2b are each expressed as a sinusoidal wave having a peak-to-peak amplitude of V.sub.in =1 mV, the potentials V.sub.2c and V.sub.2d will be sinusoidal waves of an amplitude V.sub.out =30 mV and have different polarities.
(A-4) Problem
In general, a bipolar transistor includes a parasitic capacitance between its base and its collector. In FIG. 6, parasitic capacitances in the transistors Q1 and Q2 which form the input circuit 20 of the differential amplifier 200 are indicated at C.sub.1 and C.sub.2, respectively. These parasitic capacitances create Miller effect which increases an input capacitance. Miller effect more strongly influences an amplifier element as the amplification degree of the amplifier element increases. With respect to the transistor Q1, for example, the greater the value (V.sub.2a -V.sub.2c) is, the stronger Miller effect becomes, and hence, the larger the input capacitance becomes. With respect to the transistor Q2, the input capacitance increases as the value (V.sub.2b -V.sub.2d) becomes larger. Thus, the capacitance value of an input capacitance has a dependency on a base-collector voltage V.sub.BC of a transistor.
On the other hand, as can be seen in FIG. 7, the potential V.sub.2a which appears at the base of the transistor Q1 and the potential V.sub.2c which appears at the collector of the transistor Q1 have opposite phases. Where the amplification degree of the differential amplifier is 30, an input capacitance about 31 times as large as the capacitance C.sub.1 is applied to the input terminal 2a. In a similar manner, an increased input capacitance is applied to the input terminal 2b.
The input capacitances and the coil 10 which is connected to the input terminals 2a and 2b form an LC resonator. At the resonance frequency f.sub.0, the amplification degree of the differential amplifier shows an abrupt change. FIG. 8 is a graph showing the frequency-dependency of the amplification degree. The amplification degree stays almost unchanged in a region sufficiently smaller than the resonance frequency f.sub.0, however, the frequency-dependency of the amplification degree becomes greater as approaching the resonance frequency f.sub.0. For instance, the frequency-dependency is remarkably strong around a skirt portion G.sub.p of the graph.
Where a head coil of a hard disk is used as the coil 10, the resonance frequency f.sub.0 must be set sufficiently higher than a frequency at which the polarity of the voltage which is developed across the coil 10 switches so that the amplifier will not perform amplification around the graph skirt portion G.sub.p. However, when the resonance frequency f.sub.0 of the LC resonator falls due to increases in the capacitance values of the input capacities, the graph skirt portion G.sub.p reaches almost the frequency of the voltage which is developed across the coil 10 (including a 3-order or higher harmonics), thereby intensifying the frequency-dependency of the amplification degree.
Particularly when the voltage which is induced across the coil 10 is very small, the transistors Q1 and Q2 need be large, and therefore, the parasitic capacitances C.sub.1 and C.sub.2 are inevitably large. Hence, the input capacitances grow largely due to Miller effect, resulting in a stronger distortion in an output signal.
(A-5) First Approach to the Problem
A differential amplifier which suppresses Miller effect at the transistors Q1 and Q2 has been proposed, aiming at reducing the input capacitances in the input circuit 20 and processing a week input signal without much distorting the same.
FIG. 9 is a circuitry diagram showing a structure of such a differential amplifier 201. The differential amplifier 201 additionally comprises a bias circuit 30 between the input circuit 20 and the output circuit 50, but is otherwise the same as the differential amplifier 200.
The bias circuit 30 includes NPN transistors Q3 and Q4 which each receive at its base a potential V.sub.B from a power source. Emitters of the transistors Q3 and Q4 are connected to the collectors of the transistors Q1 and Q2 of the input circuit 20, while collectors of the transistors Q3 and Q4 are connected to the output terminals 2c and 2d, respectively. In other words, between the power source terminal 3 and the ground GND, the resistor 11, the transistor Q3, the transistor Q1 and the constant current source 1a are connected in series in this order. It is also between the power source terminal 3 and the ground GND that the resistor 12, the transistor Q4, the transistor Q2 and the constant current source 1a are connected in series in this order.
FIG. 10 is a waveform diagram showing potentials at the respective parts of the differential amplifier 201. Where the amplification degree of the differential amplifier 201 is approximately 30 and the potentials V.sub.2a and V.sub.2b supplied respectively to the input terminals 2a and 2b are 180 degrees out of phase from each other and are each a sinusoidal wave which have a peak-to-peak amplitude of V.sub.in =1 mV, the potentials V.sub.2c and V.sub.2d change 180.degree. out of phase from each other as shown in FIG. 10, manifesting themselves each as a sinusoidal wave having a peak-to-peak amplitude of V.sub.out =30 mV. Disregarding a base current which flows through the transistor Q3, collector currents flowing through the transistors Q1 and Q3 are equal to each other.
On the other hand, a base-emitter voltage V.sub.BE3 of the transistor Q3 changes in a similar manner to the base-emitter voltage V.sub.BE1 of the transistor Q1 as described later. Since a potential at the base of the transistor Q3 is fixed at the potential V.sub.B, a potential V.sub.3 at the emitter of the transistor Q3 (i.e., at the collector of the transistor Q1) changes 180.degree. out of phase from the potential V.sub.2a at an amplitude which is equal to V.sub.in. Hence, a variation in the base-collector voltage of the transistor Q1 is limited to the range of .+-.1 mV. Likewise, a potential V.sub.4 at the collector of the transistor Q2 changes out of phase from the potential V.sub.2b, and a fluctuation in the base-collector voltage of the transistor Q2 is limited to the range of .+-.1 mV.
From the time t.sub.0 to the time t.sub.1, the potential V.sub.2a rises 0.5 mV and the potential V.sub.2b drops 0.5 mV. This increases the emitter current flowing through the transistor Q1 and reduces the emitter current flowing through the transistor Q2 as in the differential amplifier 200. As a result, the base-emitter voltage V.sub.BE3 of the transistor Q3 increases by 0.5 mV while the base-emitter voltage V.sub.BE4 of the transistor Q4 decreases by 0.5 mV. Hence, the base potential of the transistor Q1 increases by 0.5 mV while the collector potential of the transistor Q1 drops by 0.5 mV so that the base-emitter voltage V.sub.BE1 of the transistor Q1 falls by 1 mV. At the same time, the base potential of the transistor Q2 drops by 0.5 mV while the collector potential of the transistor Q2 increases by 0.5 mV so that the base-emitter voltage V.sub.BE2 of the transistor Q2 increases by 1 mV. Consequently, if the potentials V.sub.2a and V.sub. 2b are sinusoidal waves, the polarities will change and the base-emitter voltages of the transistors Q1 and Q2 will vary within the range of .+-.1 mV.
The reason why the base-emitter voltage of the transistor Q3 changes in a similar manner to the base-emitter voltage of the transistor Q1 can be explained as follows using equations. Disregarding the base current, and assume that a current I.sub.0 =I.sub.a /2 flows between the collector and the emitter in each one of the transistors Q1 and Q2 when the potentials V.sub.2a and V.sub.2b given to the bases of the transistors Q1 and Q2 are equally V.sub.0 and that the emitter potentials of the transistors Q1 and Q2 are both V.sub.Q. Under such circumstances, when the potentials V.sub.2a and V.sub.2b become V.sub.0 +.DELTA.V and V.sub.0 -.DELTA.V, respectively, the potential V.sub.Q remains unchanged and the emitter current in the transistor Q1 increases to I.sub.0 +.DELTA.I while the emitter current in the transistor Q2 decreases to I.sub.0 -.DELTA.I since the transistors Q1 and Q2 have the same structure. Since the potential V.sub.Q does not change, a change in the base-emitter voltage V.sub.BE1 of the transistor Q1 is equal to .DELTA.V, and therefore, Shockley's equation holds between the base-emitter voltage V.sub.BE1 and the emitter currents: ##EQU1##
These emitter currents are equal to each other since the transistors Q1 and Q3 are connected in series to each other. Hence, the emitter current of the transistor Q3 is I.sub.0 when the potentials V.sub.2a and V.sub.2b are equally V.sub.0, and amounts to I.sub.0 +.DELTA.I if the potentials V.sub.2a and V.sub.2b change to V.sub.0 +.DELTA.V and V.sub.0 -.DELTA.V, respectively. Therefore, from Shockley's equation, a change .DELTA.V.sub.BE3 in the base-emitter voltage of the transistor Q3 is: ##EQU2## Thus, it is found from Eqs. (1) and (2) that the change .DELTA.V.sub.BE3 is equal to the change .DELTA.V in each of the potentials V.sub.2a and V.sub.2b.
Hence, EQU V.sub.3 =V.sub.B -V.sub.BE3 EQU V.sub.3 =V.sub.Q +V.sub.BE1 +V.sub.BC1 ( 3)
From Eq. (3), Eq. (4) below is yielded. The left-hand side of Eq. 4 has a constant value. EQU V.sub.B =V.sub.Q =(V.sub.BE1 +V.sub.BE3)+V.sub.BC1 ( 4)
As described earlier, since the voltages V.sub.BE1 and V.sub.BE3 change in phase with the potential V.sub.2a by the same amount of variation .DELTA.V, a voltage V.sub.BC1 changes by an amount 2.DELTA.V 180.degree. out of phase from the potential V.sub.2a. That is, the potentials V.sub.2a and V.sub.2b change to V.sub.0 +.DELTA.V and V.sub.0 -.DELTA.V, respectively, so that the voltage V.sub.BC1 decreases by 2.DELTA.V. To the contrary in the transistor Q2, its base-collector voltage V.sub.BC2 increases by 2.DELTA.V.
Thus, in the differential amplifier 201, although the base-collector voltages V.sub.BC1 and V.sub.BC2 of the transistors Q1 and Q2 will not change as much as in the differential amplifier 200, there still clearly are fluctuations in the base-collector voltages and Miller effect remains reasonably influential. This is particularly undesirable when an input signal to the differential amplifier 201 is very week since the weaker the input signal to the differential amplifier 201 is, more greatly distorted the input signal will be by even a slight increase in the capacitance.
(A-6) Second Approach to the Problem
As measures to eliminate the influence of the distortion, the capacitance values of the parasitic capacitances C.sub.1 and C.sub.2 of the transistors Q1 and Q2 may be set small enough to be free from the influence of Miller effect. In short, this is to reduce the base-emitter contact areas of the transistors Q1 and Q2.
However, to realize this, the bases must be formed small, which naturally requires reductions in the emitter areas in the base regions. Therefore, this approach unadvantageously increases the base resistances and the emitter resistances, and will sacrifice the noise characteristics.